Linear arrays of multimode laser diodes are well known in the art and generally comprise a plurality of stripes of p semiconductor material formed over additional layers of p material and n material in a wafer which is generally mounted to a heat sink and energized with a current source to induce lasing. Generally, these multistripe arrays are useful in that they increase the amount of laser output available from a semiconductor laser, and they may also be modulated by directly modulating the injection drive current, although no amplification is achieved by this direct modulation. Furthermore, there are limits to the amount of power output which can be achieved with a semiconductor multistripe laser diode device as catastrophic failure of the device occurs due to facet damage from overdriving, or in another manner due to excessive power densities in the device. Also known in the prior art is a technique for individually addressing and energizing the individual stripes of a multistripe array. This is generally achieved by forming separate electrical contacts on the individual stripes.
In working with multistripe laser diodes, the inventors herein have succeeded in reliably producing a phenomenon known as transverse lasing in a repeatable manner. Transverse lasing is characterized by a quenching of normal lasing along the length dimension of the stripe and the creation of a lasing effect in a perpendicular direction through the width of the array. This effect has been produced in multistripe laser arrays in which the ratio of emitting stripe width to non-emitting stripe isolation region width was substantially one-to-one. In such a wafer, there is a threshold for normal lasing such as would be ordinarily expected in a multistripe laser array. In wafers configured appropriately, a corresponding threshold current for transverse lasing has been found to be approximately twice that of the normal threshold. As this transverse threshold is reached, the normal output laser is very rapidly and effectively quenched, and transverse lasing is propagated through the wafer. It has also been found that repeatedly driving the current above this transverse lasing threshold results in no change to the power/current characteristics of the device, thereby indicating that transverse lasing does not result in damage to the wafer. Furthermore, because of the rapid switching from normal to transverse lasing at the threshold, it is anticipated that this effect can be used to provide a rather high speed communication device. In the preferred embodiment, switching rates have been achieved which indicate that frequencies as high as 50 MHz may be achieved.
A further feature and advantage of this transverse lasing phenomenon is the opportunity to achieve an amplification factor. This is because the injection current can be modulated in a relatively small band about the transverse lasing threshold which will cause the laser diode array to rapidly switch from normal lasing to transverse lasing at full power. Still another application for this device would utilize the individual stripe accessing technique already known in the prior art. By separately addressing the individual stripes, a high speed, multiple input, AND-gate may readily be achieved, and other logic elements could be formed as well.
The phenomenon of transverse lasing is generally thought to be due to parasitic oscillations in the device. Parasitic oscillation in laser diode devices has been known for some time, particularly with respect to broad area devices. However, the present invention differs from broad area lasers and other devices in which parasitic oscillation has been previously observed in that the parasitic oscillation in the present device depends upon the interaction of many otherwise normal lasers occupying the same planar waveguide and, to the inventor's knowledge, has not been achieved by others. This particular wafer arrangement makes the transverse lasing threshold for parasitic oscillation a well defined and repeatable parameter rendering it capable of useful application.
While some of the principal advantages and features of the present invention have been briefly described, a fuller understanding of the invention may be obtained by referring to the drawings and description of the preferred embodiment which follow.